Radiation Sciences 4.2.1

RADIATION SCIENCES

Objectives

• At the end of this session students must be able to;
• Describe X-ray tube, tube housing, shielding, anode, cathode and tube cooling.
• Describe generation of X-rays and factors affecting its quality.

Demonstrate patient condition in deciding exposure type and dose.

X-RAY TUBE

The X-ray tube is the core component of the X-ray system, where X-rays are generated.

It is a vacuum-sealed device that accelerates electrons from the cathode to the anode, producing X-rays through the interaction of electrons with the anode material.

X-RAY TUBE

• The x-ray tube contains two principal elements:
• Filament (Cathode): boils off electrons

Target (Anode): electrons strike to produce x-rays

Additional components

• Expansion bellows (provide space for oil to expand)
• Tube envelope (evacuated)
• Tube housing
• Cooling dielectric oil
• Rotor
• Induction stator

Tube window: usually made from beryllium, not glass

The function of the X-ray tube is to:

Provide a beam of X-rays from as near a point source as possible (focus).

Dissipate the heat produced effectively to prevent damage to the X-ray tube (approximately 99 per cent of the energy conversions produce heat).

Provide a consistent quality (kVp) and quantity (mAs) of radiation

Radiation Sciences 4.2.1

Allow X-rays to emerge only from the window (port) of the housing of the tube and exclude emissions from elsewhere in the housing, which is lined with lead sheet.

Provide an electrically safe environment for the practitioner.

The tube is securely supported, but capable of easy movement into any position and then being maintained in that position.

TUBE HOUSING

TUBE HOUSING

Is the portion of an x-ray system which contains the x-ray tube and/or secondary target.

TUBE HOUSING

• The tube housing encloses the X-ray tube and serves several key functions:
• Radiation Shielding: Made of lead-lined material to prevent stray X-rays from escaping.

Mechanical Protection: Protects the fragile X-ray tube from external damage.

Electrical Insulation: Prevents high-voltage electrical leakage.

Heat Dissipation: Contains cooling systems to manage heat generated during operation.

SHIELDING

External Shielding: Lead lining in the tube housing minimizes exposure to stray radiation.

Internal Shielding: Filters may be placed to remove low-energy, non-useful X-rays, reducing patient exposure.

Collimators: Further limit the beam to the desired area, enhancing safety and image quality.

CATHODE

• The cathode emits electrons through thermionic emission when heated. It consists of:
• Filament: A coiled wire (usually tungsten) that produces electrons when heated.

Focusing Cup: Negatively charged to direct and focus the electron beam toward the anode.

Functions:

• Provides a source of electrons needed for X-ray production.

Shapes and focuses the electron stream for efficient X-ray generation.

ANODE

• The anode is the target where high-speed electrons collide, producing X-rays. It can be:
• Stationary Anode: Used in low-power X-ray systems (e.g., dental X-rays).

Rotating Anode: Used in high-power systems for improved heat dissipation.

STATIONARY AND ROTATING ANODE

Materials in Anode:

• Tungsten: High atomic number and melting point, making it ideal for X-ray production.

Molybdenum or Copper: Sometimes used as a base for the rotating anode.

Why Tungsten for Target?

Atomic number -Tungsten’s high atomic number, 74, results in high-efficiency x-ray production and in high-energy x-rays. Thermal conductivity -Tungsten has a thermal conductivity nearly equal to that of copper. It is therefore an efficient metal for dissipating the heat produced.

High melting point, Any material, if heated sufficiently, will melt and become liquid. Tungsten has a high melting point (3400°C compared with 1100°C for copper) and therefore can stand up under high tube current without pitting or bubbling.

Functions:

• Converts kinetic energy of electrons into X-rays (about 1% efficiency).

Dissipates heat generated during the process.

TUBE COOLING

Cooling is essential due to the immense heat generated during X-ray production. Methods include:

• Oil Cooling: Oil surrounds the tube to absorb and transfer heat away from the X-ray tube.

Air Cooling: Fans circulate air around the housing to dissipate heat.

Rotating Anode: Distributes heat over a larger surface area, increasing thermal efficiency.

Water Cooling: Used in high-end systems for superior heat removal.

GENERATION OF X-RAYS

X-rays are produced when high-speed electrons collide with a metal target (anode) in an X-ray tube.

The process involves the conversion of kinetic energy of electrons into electromagnetic radiation.

Below is a step-by-step explanation of how X-rays are generated:

• Electron Production (Thermionic Emission)

Cathode: The cathode contains a filament (usually tungsten) that is heated by an electrical current.

Electron Emission: When heated, the filament releases electrons through thermionic emission, where the thermal energy overcomes the binding energy of electrons.

2. Acceleration of Electrons

High Voltage (kVp): A high voltage is applied between the cathode (negative) and anode (positive), creating a strong electric field.

Electron Acceleration: The electrons are rapidly accelerated toward the anode, gaining kinetic energy in the process.

3. Electron-Anode Interaction

When the high-speed electrons hit the anode (usually tungsten due to its high atomic number and melting point), two main processes occur:

• Bremsstrahlung Radiation (Braking Radiation)
• Mechanism: Electrons are decelerated by the nuclear field of tungsten atoms.

Energy Conversion: The loss of kinetic energy is emitted as X-ray photons.

b) Characteristic Radiation

Mechanism: High-energy electrons eject inner-shell electrons (usually K-shell) from the tungsten atom.

Energy Release: Outer-shell electrons fill the vacancy, releasing energy in the form of X-rays.

4. X-ray Beam Formation

Primary Beam: The X-rays produced are directed toward the patient or image receptor.

Filtration: Low-energy X-rays are removed by an aluminum filter to reduce patient dose and improve beam quality.

Collimation: The beam is shaped and limited by a collimator to target the area of interest.

5. Energy Distribution

Efficiency: Only about 1% of the electron energy is converted into X-rays; the remaining 99% is dissipated as heat.

Cooling: The anode is cooled by rotating it or using oil or water cooling systems to manage the heat.

FACTORS AFFECTING THE QUALITY OF X-RAY GENERATED

The quality of X-rays generated in an X-ray tube is influenced by several factors that affect beam energy, intensity, and consistency.

These factors determine the contrast, sharpness, and overall diagnostic value of the X-ray image.

1. Tube Voltage (kVp – Kilovoltage Peak)

• The voltage applied across the X-ray tube between the cathode and anode.
• Effect:

Higher kVp increases the energy of X-rays, improving penetration.

Affects contrast: Higher kVp results in lower image contrast (long grayscale), while lower kVp produces higher contrast (short grayscale).

Influences the proportion of Bremsstrahlung and Characteristic radiation.

2. Tube Current (mA – Milliamperage)

• The current that flows through the filament, controlling the number of electrons emitted.
• Effect:

Determines the quantity of X-rays produced (not their energy).

Higher mA produces a higher intensity X-ray beam, resulting in a darker image (greater density).

3. Exposure Time (s)

• The duration for which the X-ray exposure is active.
• Effect:
• Longer exposure time allows more X-rays to be produced, increasing image density.

Shorter times reduce motion blur but may require higher mA to maintain image quality.

4. Filament Temperature

• Controlled by the tube current, affecting electron emission from the cathode.
• Effect:
• Higher filament temperature produces more electrons, increasing X-ray output.

Overheating can reduce tube lifespan and image quality.

5. Focal Spot Size

• The size of the area on the anode where electrons strike.

Effect:

Smaller focal spots produce sharper images with better spatial resolution but may overheat.

Larger focal spots allow for higher output but reduce sharpness, causing blurring

6. Target Material (Anode Composition)

• Common Materials: Tungsten, molybdenum, or rhodium.

Effect:

Higher atomic number (Z) materials like tungsten produce more efficient X-ray generation due to greater Bremsstrahlung radiation.

Different materials produce characteristic X-rays with different energies, affecting contrast.

7. Anode Angle

Effect on X-ray Distribution: The anode angle affects the effective focal spot size and the intensity distribution.

Impact on Image: A steeper angle produces a smaller effective focal spot, enhancing resolution.

8. Anode Rotation Speed (for Rotating Anodes)

• Speed at which the anode rotates to distribute heat.
• Effect:
• Faster rotation improves heat dissipation, allowing higher exposures without overheating.

Slower speeds can lead to localized heat damage and reduced tube life.

9. Filtration

• Hardening of X-ray beam by removal of low-energy X-rays.,
• Use of metal filters (e.g., aluminum)
• Effect:
• Increases the average energy of the X-ray beam (beam hardening).

Reduces patient dose and improves contrast by eliminating non-diagnostic X-rays

10. Tube Cooling System

• Oil cooling, air cooling, water cooling, or rotating anode.
• Effect:
• Efficient cooling prevents overheating, which can degrade image quality.

Insufficient cooling can cause focal spot damage, reducing image sharpness.

11. Beam Collimation and Alignment

• Devices used to shape and direct the X-ray beam to the desired area.
• Effect:
• Reduces scatter radiation, enhancing contrast.

Ensures uniform exposure, improving image consistency.

12. Vacuum Quality in the Tube

• The vacuum inside the X-ray tube prevents interaction of electrons with air molecules.
• Effect:
• A good vacuum ensures efficient electron travel from cathode to anode.

A compromised vacuum reduces X-ray production efficiency and tube lifespan.

PATIENT CONDITION IN DECIDING EXPOSURE TYPE AND DOSE

Determining the appropriate exposure type and dose for X-ray imaging depends on the patient's condition, including factors like body size, age, pathology, and clinical indications.

1. Patient Size and Body Composition

• The amount of tissue X-rays must penetrate affects the exposure needed.
• Pediatric Patients

Considerations: Children are more sensitive to radiation.

Exposure Type: Use low-dose settings to minimize radiation exposure.

Technique Adjustments:

• Lower kVp and mAs to reduce dose.
• Use faster digital detectors with high sensitivity.
• Tight collimation to reduce exposed area.

Apply additional filtration to remove low-energy X-rays.

b) Obese Patients

• Considerations: Greater tissue thickness requires more X-ray penetration.
• Exposure Type: Higher exposure to ensure adequate penetration and image quality.

Technique Adjustments:

• Increase kVp (to improve penetration).
• Increase mAs (to maintain image brightness).

Use a larger focal spot to handle higher heat production.

2. Pathological Conditions

Different pathologies require modifications to exposure settings based on tissue density or abnormalities.

• Pulmonary Conditions (e.g., Pneumonia, COPD)
• Pneumonia (increased lung density):
• Technique: Slightly increase mAs to compensate for increased density.

COPD (decreased lung density):

Technique: Reduce mAs to avoid overexposure and maintain contrast.

b) Bone Imaging (e.g., Osteoporosis, Fractures)

• Osteoporosis (reduced bone density):
• Technique: Lower kVp to enhance contrast between low-density bone and surrounding tissue.
• Fractures (dense bone imaging):

Technique: Use moderate kVp and mAs to ensure detailed imaging of the fracture line.

3. Age and Sensitivity

• Older patients and those with specific medical conditions may require dose adjustments.
• Elderly Patients

Considerations: Older tissues may be less dense, but cumulative exposure should be minimized.

Technique Adjustments:

• Use lower mAs to reduce dose while maintaining diagnostic quality.

Lower kVp may enhance contrast for detecting fine details like fractures.

b) Pregnant Patients

• Considerations: Minimize fetal exposure.

Technique Adjustments:

• Only perform X-rays if medically necessary.
• Use protective shielding (e.g., lead aprons) for the abdomen.

Optimize exposure by lowering mAs and collimating the beam.

4. Imaging Region and Type of Examination

• Different body regions require different exposure parameters based on tissue composition.
• Chest X-ray

Considerations: Air-filled lungs allow lower exposure.

Technique: Use high kVp (~100–120 kVp) for lower contrast and better penetration, with low mAs.

b) Abdominal X-ray

• Considerations: Denser organs require more penetration.

Technique: Use moderate kVp (~70–80 kVp) and higher mAs to enhance soft tissue detail.

c) Extremity X-ray (e.g., hands, feet)

• Considerations: Thin tissues require minimal exposure.

Technique: Use low kVp (~50–60 kVp) and low mAs for optimal bone contrast.

5. Use of Contrast Media

• Contrast media alters tissue density and affects exposure settings.
• Barium Studies (e.g., GI tract imaging):
• Increase kVp (typically 100–120 kVp) to penetrate the dense contrast material.
• Iodine-based Contrast (e.g., angiography):

Use moderate-to-high kVp to visualize the contrast-filled vessels clearly.